Fracture Toughness and Subcritical Crack Growth in Polycrystalline Silicon

The fracture behavior of polycrystalline silicon in the presence of atomically sharp cracks is important in the determination of the mechanical reliability of microelectromechanical system (MEMS) components. The mode-I critical stress intensity factor and crack tip displacements in the vicinity of atomically sharp edge cracks in polycrystalline silicon MEMS scale specimens were measured via an in situ atomic force microscopy/digital image correlation method. The effective (macroscopic) mode-I critical stress intensity factor for specimens from different fabrication runs was $1.00±0.1MPa√m$⁠, where $0.1MPa√m$ is the standard deviation that was attributed to local cleavage anisotropy and grain boundary effects. The experimental near crack tip displacements were in good agreement with the linearly elastic fracture mechanics solution, which supports $K$ dominance in polysilicon at the scale of a few microns. The mechanical characterization method implemented in this work allowed for direct experimental evidence of incremental (subcritical) crack growth in polycrystalline silicon that occurred with crack increments of $1-2μm$⁠. The variation in experimental effective critical stress intensity factors and the incremental crack growth in brittle polysilicon were attributed to local cleavage anisotropy in individual silicon grains where the crack tip resided and whose fracture characteristics controlled the overall fracture process resulting in different local and macroscopic stress intensity factors.